Field of the Invention
The invention relates to a fully integratable voltage controlled oscillator (VCO) circuit for producing sinusoidal UHF oscillations, including bipolar transistors, a tuned circuit having an integrated spiral inductance connected in parallel with an integrated varactor having a pn junction acted on by a VCO control voltage governing a resonant frequency, and a cross-coupled so-called NIC (Negative Impedance Converter) circuit transforming a negative resistance into the tuned circuit and counteracting damping of the tuned circuit.
In order to prepare a voice/data signal for optimized transmission using a radio transmission channel (radio), the voice/data signal is modulated onto a so-called radio frequency (RF) carrier. In many applications, the RF carrier is formed of sinusoidal oscillations which are produced by a voltage controlled oscillator (VCO).
Depending on the field of application, for example in the DECT (Digital European Cordless Telecommunications) system (which is transmitted by using the UHF frequency band), GSM (Global System for Mobile Communications) system or in similar systems, the standards which exist for those systems place particular requirements on the voltage controlled oscillator. One of the most important requirements is that the so-called phase noise can be kept low since, for example, that limits the maximum power which can be transmitted in adjacent channels (in the transmission mode) or the maximum power from adjacent channels in the channel being used (in the receiving mode).
Those requirements are generally so stringent that, to date, it has been virtually impossible to produce fully integrated voltage controlled oscillator (VCO) circuits which comply, for example, with the DECT Specification. The critical parameter in the DECT Standard in that case is the VCO phase noise at an offset of 4.7 MHz (third adjacent channel). According to the specification, that is -132 dBc/Hz.
Three integrated voltage controlled UHF oscillator circuits which could, to some extent, satisfy the DECT Specifications are already known from the literature.
The first circuit is based on an article entitled "Silicon Bipolar VCO Family for 1.1 to 2.2 GHz with Fully-Integrated Tank and Tuning Circuits", by Bart Jansen, Kevin Negus and Don Lee, in 1997 IEEE International Solid-State Circuits Conference, ISSCC97/Session 23/Analog Techniques/Papers SP 23.8, Feb. 8, 1997, pages 392 and 393. In that known integrated VCO oscillator, a tuned circuit which includes an integrated spiral inductance connected in parallel with an integrated varactor has its damping counteracted by a circuit which transforms a negative resistance into the tuned circuit. Such a circuit is generally called an NIC (Negative Impedance Converter) circuit.
A control voltage for the VCO is applied to the pn junction of the varactors, through the use of which the total capacitance of the tuned circuit, and thus the oscillation frequency, can be varied. That VCO is formed by bipolar transistors. The advantage of that type of transistor is its low 1/f noise, which is advantageous, particularly for setting operating points.
The disadvantage is the high gradient "gm" (Id/Ut) and the base path resistance of the bipolar transistors. When the NIC circuit passes through its "zero crossing", that is to say both transistors in the current switch are switched on, a relatively large "noise charge packet" is introduced into the tuned circuit and makes itself evident in a disadvantageous manner as phase noise.
The second circuit which is known in that context originates from an article entitled "A Balanced 1.5 GHz Voltage Controlled Oscillator with an Integrated LC Resonator", by Leonard Dauphinee, Miles Copeland and Peter Schvan, in "1997 IEEE International Solid-State Circuits Conference, ISSCC97/Session 23/Analog Techniques/Papers SP 23.7, Feb. 8, 1997", pages 390 and 391, and is based on a differential Colpitts oscillator. That circuit is also formed by bipolar transistors and therefore suffers from the same problem, which has already been described in conjunction with the first known VCO circuit.
The third VCO circuit which is known in that context is based on an article entitled "A 1.8-Ghz CMOS Low-Phase-Noise Voltage-Controlled Oscillator with Prescaler", by Jan Craninckx and Michel S. J. Steyaert, in "IEEE Journal of Solid-State Circuits", Vol. 30, No. 12, December 1995, pages 1474 to 1482 and, in contrast to the first two integrated VCO circuits dealt with above, is formed from CMOS transistors.
A bonding wire inductance is used in the resonant circuit, and that forms a weakness of that technique since the inductance value cannot be reproduced well. The circuit for counteracting the damping of the tuned circuit is not described in any more detail in the article. It can be assumed that an NIC type is also used therein.
The disadvantage of CMOS transistors is the relatively high 1/f noise of that type of transistor. The limit frequency of 1/f noise in CMOS transistors is typically in the MHz region, while in contrast it is in the kHz region for bipolar transistors. The advantage of the CMOS transistor is the relatively low gradient "gm" which makes the noise produced at the current switch changeover time negligibly low.